This invention relates to optical isolators.
An optical isolator allows light to pass through it substantially unattenuated in one direction (forward), while highly attenuating the intensity of light passing through it in the other direction (reverse). Optical isolators are therefore used to isolate light sources, typically lasers, from reflections and to prevent parasitic oscillations in high gain optical amplifier chains. For example, single-frequency laser diodes require greater than 60 dB isolation in coherent optical communications and optical sensing applications.
A Faraday rotator is commonly used as a key component to provide the isolation. A Faraday rotator has the property that it rotates the polarization vector of light passing through it in the same sense or chirality (clock-wise or counter-clock-wise) regardless of the direction in which the light passes through the Faraday rotator. For example, if a Faraday rotator were constructed to rotate the polarization vector of the light passing through it in a given direction, for example clock-wise by 45.degree., passing that light back through the Faraday isolator in the opposite direction would not cause the polarization vector of the light to be rotated counter-clockwise by 45.degree. back to its original orientation. Instead, the polarization vector of the light would be rotated an additional 45.degree. clock-wise, resulting in light whose polarization vector would then be perpendicular to its original orientation.
This behavior differs from the behavior of other optically active rotators, such as quartz, in which the sense or chirality of rotation of the polarization vector of light is determined by the direction of the light as it passes through the optically active rotator. Therefore, if the polarization vector of light passing through quartz in a given direction is rotated, for example clock-wise 45.degree., passing that light back through the quartz rotator in the reverse direction will result in the polarization vector of the light being rotated counter-clock-wise 45.degree. back to its original orientation.
A Faraday rotator is constructed by placing a magneto-optical material within a magnetic field. When such a material is exposed to a magnetic field, the material causes the polarization vector of light passing through it to rotate. The amount of rotation is a function of the material itself, the wavelength of the light passing through the rotator, the applied magnetic field strength and the ambient temperature of the material.
Typically an optical isolator is constructed by placing a 45.degree. Faraday rotator, that is, a Faraday rotator which rotates the plane of polarization by 45.degree., between two polarizers oriented such that their polarization axes are at 45.degree.. The orientation of the light passing through the first polarizer is rotated 45.degree. upon passing through the Faraday rotator in the forward direction. Since the second polarizer is oriented at 45.degree. to the first polarizer, light with that new polarization can pass through. Light passing in a reverse direction through the second polarizer undergoes an additional 45.degree. rotation upon passing through the Faraday rotator. This results in its polarization vector of the light being oriented 90.degree. to the orientation of its polarization vector in the forward direction. This additional rotation prevents the light from passing through the first polarizer in the reverse direction.
Dispersion causes the amount of rotation produced by a Faraday rotator to be wavelength dependent. Because of this wavelength sensitivity, a typical isolator provides good isolation over a narrow band of wavelengths and the isolator generally needs to be tuned to provide good isolation over a wide range of wavelengths. That is, for example, if the Faraday rotator were constructed to rotate the polarization of light at wavelength .lambda..sub.1 by 45.degree., at .lambda..sub.2 some wavelength away from .lambda..sub.1, the Faraday rotator might only rotate the polarization 32.degree.. Therefore the light passing through the rotator in the reverse direction would only be rotated 32.degree. relative to the polarization selected by the second polarizer. The polarization of this light is oriented 77.degree. away from the orientation selected by the first polarizer. Of light propagating in the reverse direction a fraction, equal to cos.sup.2 (77.degree.)=0.05, would be transmitted by the first polarizer, corresponding to 13 dB of isolation. In order to provide isolation at .lambda..sub.2, the Faraday rotator would have to be adjusted to produce a 45.degree. rotation at .lambda..sub.2 or the polarizer would have to be rotated, but in doing either, the isolator would lose the ability to isolate at .lambda..sub.1. Most currently produced optical isolators will provide good isolation over a narrow wavelength range, for example, greater than 30 dB over 30 nm. Several methods may be used to tune an isolator, that is, switch the wavelength of maximum isolation from one wavelength to another. The amount of Faraday rotation can be changed by: varying the magnetic field applied to the rotator; rotating one of the polarizers to a new orientation; or altering the temperature of the rotator. This tuning of wavelengths takes time (one second or longer) and is therefore inappropriate when a rapid switching of wavelengths is desired or when simultaneous isolation at many different wavelengths is required.
Iwamura and co-workers have reported an isolator ("A compact optical isolator using a Y.sub.3 Fe.sub.5 O.sub.12 crystal for near infra-red radiation", Optical and Quantum Electronics, 10 (1978) p. 393) made by placing a 45.degree. YIG Faraday rotator adjacent to a 45.degree. optically active quartz rotator and placing both between a pair of 90.degree. crossed polarizers. Since their device was only partially dispersion compensated, it provided 23 dB of isolation only over a wavelength range of 1.1-1.4 .mu.m.
Johnston and Proffit (IEEE Journal of Quantum Electronics, QE-16, (1980) p. 483) reported a dispersion compensated unidirectional device for a ring-laser having a Faraday rotator in conjunction with an optically active rotator to give a net 0.degree. rotation in the forward direction and a small rotation in the reverse direction. Although this device used dispersion compensation to work over a broad wavelength range, it was not an isolator.